Electrowetting display having controlled fluid motion

ABSTRACT

An electrowetting display device comprises a plurality of picture elements ( 2 ) having a first support plate ( 5 ) including a surface ( 68 ) and a second support plate. A space ( 10 ) of a picture element between the surface of the first support plate and the second support plate includes at least one first fluid and a second fluid immiscible with each other, the second fluid being electroconductive or polar. The first support plate includes an electrode ( 46 ) for applying an electric field in the picture element. It also includes a layer ( 44 ) arranged on a side of the electrode facing away from said space. The layer forms an electronic component ( 33 ) for applying a voltage to the electrode. The electrode comprises a height difference ( 86 ) corresponding to a thickness of the layer ( 44 ). The height difference causes an inhomogeneous electric field in the space, providing a preferred direction of motion of the first fluid on application of the electric field.

CROSS REFERENCE TO RELATED APPLICATIONS

This Patent Application is a continuation of International PatentApplication No. PCT/EP2008/066942 filed on Dec. 5, 2008, entitled,“ELECTRO WETTING DISPLAY HAVING CONTROLLED FLUID MOTION,” which claimspriority to GB 0723861.1 filed on Dec. 6, 2007, the contents andteachings of which are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to an electrowetting display comprising aplurality of picture elements having a first support plate and a secondsupport plate and a space between the first support plate and the secondsupport plate, the space including at least one first fluid and a secondfluid immiscible with each other.

BACKGROUND OF THE INVENTION

Display devices containing fluids which can be switched betweendifferent fluid configurations to provide different opticalcharacteristics are known. Several measures have been proposed tocontrol the motion of the fluids within a picture element of the displaydevice.

International patent application WO 2003/071346 describes such a displaydevice. A movement of the fluids during switching is controlled by usingan insulating layer on the first support plate having a particular shapeto modify an electric field applied to the fluids. A special electrodestructure or an inhomogeneous oil fluid layer may be used alternativelyto control the fluid motion.

International patent application WO 2006/021912 describes a devicehaving picture elements which have a shape that determines, uponswitching, a preferred direction of movement of fluids within thepicture elements.

International patent application WO 2004/104671 describes an opticalswitch containing fluids in which the shape of an electrode used toapply the voltage determines a preferred direction of movement of thefluids upon switching.

International application WO 2007/141218 describes a display devicehaving a protruding hydrophilic part on an otherwise flat first supportplate. The hydrophilic part operates as oil movement initiator. Adisadvantage of this known display is that the arrangement of theprotruding part requires an additional process step during themanufacture of the display.

It is an object of the present invention to provide a display device inwhich the control of the fluid motion can be made in a simpler manner.

SUMMARY OF THE INVENTION

The object of the invention is achieved by an electrowetting displaydevice comprising a plurality of picture elements having a first supportplate and a second support plate and a space between the first supportplate and the second support plate, the space including at least onefirst fluid and a second fluid immiscible with each other, the secondfluid being electroconductive or polar; the first support plateincluding at least one electrode for applying an electric field in thepicture element and a layer arranged on a side of the at least oneelectrode facing away from said space, the layer forming an electroniccomponent for applying a voltage to the at least one electrode; whereinthe at least one electrode comprises a height difference correspondingto a thickness of the layer.

The field strength in the first fluid will be higher where the layer ofthe first fluid is thinner and smaller where the layer is thicker. Thechanges in height of the one or more electrodes will therefore cause achange in the strength of the electric field over the layer of firstfluid. On applying a voltage to the one or more electrodes, the firstfluid will move in a direction of lowest electric field. The shape ofthe height difference, i.e. the pattern of the height difference as seenfrom the space of the picture element, determines a preferred directionof motion of the first fluid.

The height difference in the at least one electrode is caused by a layerthat forms an electronic component for applying a voltage to the atleast one electrode. Usually the layer and the electrode are part of astack of layers arranged on a substrate, the stack and substratetogether forming the first support plate. The layer may form one or moreelectronic components for applying the voltage. An electronic componentmay be formed by one layer or a plurality of layers.

The desired shape of the height difference may be obtained by anappropriate shape of the electrode and/or an appropriate choice shape ofthe patterning of the layer, thereby determining the preferred directionof motion of the first fluid. Hence, the layer has the function of bothforming an electronic component and creating height differences for theat least one electrode. Since no layers need be arranged in the firstsupport plate specifically for creating the height difference of theelectrode, the manufacture of the display device can be simplified.

The layer may be arranged on a substrate of e.g. plastic or glass, so asto cause one or more height differences of the at least one electrode.The correspondence of the height difference of the at least oneelectrode and the thickness of a layer may reside in the fact thatheight differences of the electrode are copied from height differencesin a patterning of the layer onto which the electrode is arranged, or inthe fact that the layer is arranged between two electrodes, therebydetermining the height difference between the electrodes. When the layerhas a pattern including height differences of for example 300 nm, thecopying may result in height difference in the electrode of for example200 nm. The copying may be made by applying the electrode directly onthe patterned layer or by applying the electrode on one or moreintermediate layers, which in turn are applied on the patterned layer.Copying may reduce the height differences because of levelling duringapplication of a layer on a patterned layer.

The height difference is determined in a direction perpendicular to aplane defined by the highest points of the at least one electrode of aplurality of picture elements. It should be noted that the heightdifference relates to the surface of the at least one electrode closestto the space of the picture element, because the position of thissurface determines the electric field strength in the space.

The at least one electrode in the first support plate is a conductinglayer such that a voltage applied between the second fluid and theelectrode causes an electric field extending from the second fluid tothe electrode.

In a special embodiment of the display device the at least one electrodecomprises a first part and a second part and the height difference isbetween the first part and the second part. When the electrode isapplied as a layer on another layer having a height difference, theelectrode may copy at least part of the height difference of the otherlayer. Although both parts of the electrode will be at the samepotential, the electric field generated by the two parts will bedifferent because of the difference in height.

In another special embodiment the picture element comprises at least twoelectrodes and the height difference is between the two electrodes. Theelectric field generated by the two electrodes is now determined by theheight difference and the voltages that can be applied to theelectrodes. The electrode itself may also have a height difference as inthe previous special embodiment.

The height difference has advantageously a shape corresponding to apattern of the layer. The layer is used for forming electroniccomponents for applying a voltage to the at least one electrode. Whenthe layer is applied and patterned on a substrate, in addition to thepattern necessary for the electronic components, extra patterns may beformed in the layer without additional effort. These extra patterns, notforming electronic components, may also be used to create patterns thatshape the height differences in the at least one electrode.

When a layer is stated to form an electronic component, the layer may bethe only constituent of the electronic component, such as a controlline, or the layer may be one of the constituents of the electroniccomponent, such as a source of a transistor.

A special embodiment includes a height difference of the at least oneelectrode that corresponds to a part or pattern of the layer that formsthe electronic component. In this embodiment the height difference isdue to the one or more electronic components and not to further patternsin the layer.

In a special embodiment the electronic component includes any of anelectrode, a control line, a transistor and a capacitor. The transistorand capacitor are electronic components typically used in pictureelements of so-called active-matrix display devices. When a firstelectrode is arranged on part of a second electrode, the electrodes willshow a height difference. In such a case the second electrode is both anelectrode and an electronic component for applying a voltage on thefirst electrode.

In an advantageous embodiment the extent of the picture element isdefined by walls arranged in the first support plate and one or more ofthe control lines being arranged inside the picture element adjacent toone of the walls. The increased height of the electrode near the wallwill increase the electric field, preferentially moving the first fluidaway from the wall. This reduces pinning of the first fluid near thewall.

When the control line is made thicker to enhance the height difference,its resistance reduces, which may be advantageous for the operation ofthe electronic components. When the control line is made thicker, itswidth may be reduced. Hence, while maintaining the resistance of thecontrol line, its width as viewed from the space of the picture elementcan be made smaller, thereby reducing its impact on the opticalperformance of an electrowetting display.

The one or more height differences of the electrode or electrodes may bearranged such that they form a recess in a corner of the pictureelement. The reducing height of the electrode will generate an electricfield on application of a voltage that causes the first fluid to movepreferentially towards the recess. The recess is an area of the surfacehaving a minimum height, the height of the surface being viewed in thedirection of the space. The minimum height may be formed by arrangingthe electronic components in parts of the picture element other than therecess.

The electrode may be arranged in an area of the picture elementdifferent from the recess, thereby reducing the field strength in therecess, compared to the rest of the picture element area. The recess mayinclude a further electrode to allow the application of a low voltagecompared to the voltage of the second fluid to reduce fringing fields atthe edge of said at least one electrode. In a preferred embodiment, thevoltage applied to the further electrode is equal to the voltage appliedto the second fluid.

The absence of the electrode in the recess may be used to form adepression in the surface of a cover layer in this area. The depressionwill cause a preferred flow of the first fluid to this corner.

The first support plate may include a further layer other than saidlayer, and the at least one electrode comprising a height differencecorresponding to a thickness of the further layer. In addition to heightdifferences caused by layers forming electronic components, theelectrode may also have height differences caused by layers not used forapplying a voltage to the electrode. These further layers may bepatterned.

In a special embodiment of the display device the first support plateincludes a conducting layer arranged on a side of the electrode facingaway from said space, the electrode and the conducting layer beingseparated by an insulating layer, and at least one through-connectionbetween the electrode and the conducting layer, the through-connectionforming a pinning location for the first fluid.

The through-connection, also called a ‘via’, causes a depression in thesurface of the electrode. The increased thickness of the layer of firstfluid and the consequential lower electric field at the position of thethrough-connection create a location where the first fluidpreferentially moves towards. The first fluid may also be attracted tothe position because of cohesion effects.

Advantageously the electrode has a recess in an area of the pictureelement and the through-connection is arranged close to or in the area.The arrangement of the area of low electric field and thethrough-connection together will increase the preference of the firstfluid to move to this area.

In a preferred embodiment of the display device a cover layer isarranged between the at least one electrode and the space of theelectrowetting element. The cover layer may be insulating and havespecific electrowetting properties, such as a hydrophilic or hydrophobicproperty.

When the cover layer is of substantially equal thickness, the surface ofthe cover layer adjoining the first and/or second fluid, will showsimilar height differences as the electrode or electrodes. The heightdifferences will cause a difference in thickness of the layer of firstfluid adjoining the surface in the state without an electric field.Changes in the thickness of the cover layer will not affect the electricfield when the dielectric constant of the material of the cover layerand the first fluid is the same. A local depression in the surface ofthe cover layer adjoining the fluids may act as a pinning location, asit requires a larger force to move the first fluid from this locationthan from a flat surface. The pinning effect may be enhanced by areduced electric field caused by the depression in the electrode.

The invention also relates to an electrowetting display devicecomprising a plurality of picture elements having a first support plateincluding a surface and a second support plate and a space between thesurface of the first support plate and the second support plate, thespace including at least one first fluid and a second fluid immisciblewith each other, the second fluid being electroconductive or polar; thefirst support plate including an electrode for applying an electricfield in the picture element and covering a first area of the pictureelement; and a storage capacitor for maintaining a voltage on theelectrode, the storage capacitor having two plates; wherein at least oneof the plates of the capacitor covers a different, second area of thepicture element.

When a voltage is applied to the electrode, an electric field will begenerated over the first area. The plate of the capacitor extending overthe second area may be set at a voltage to improve control of the motionof the first fluid. The capacitor has a double function in that it isused for maintaining a voltage on the electrode and for controlling themotion directly by generating an electric field over the second area.Preferably the plate is set at the same voltage as the second fluid,thereby reducing fringing fields at the edge of the electrode andforcing the first fluid towards the second area on application of avoltage on the electrode.

The second area is preferably arranged in a corner of the pixel element,reducing the obstruction of the field of view by the first fluid whenthe first fluid is retracted by application of the voltage.

The advantage of the display device having a plate of the capacitor inthe second area is achieved in display devices having height differencesin the surface of the first support plate and in display devices havingno such height differences in the surface.

In a special embodiment the first support plate includes a secondelectrode covering at least part of the second area and is preferablyconnected to the at least one of the plates of the capacitor by athrough-connection. The second electrode covering at least part of thesecond area may be set at a voltage to improve control of the motion ofthe first fluid. Preferably the plate is set at the same voltage as thesecond fluid, thereby reducing fringing fields at the edge of theelectrode and forcing the first fluid towards the second area onapplication of a voltage on the electrode.

The through-connection to the said plate of the capacitor may be made toform a depression in the surface of the first support plate, therebycreating a preferred motion of the first fluid towards the second area.

The at least one of the plates of the capacitor is preferably at thesame voltage as the second fluid. This reduces the electric field abovethe capacitor.

The invention further relates to an electrowetting display devicecomprising a plurality of picture elements having a first support plateand a second support plate and a space between the first support plateand the second support plate, the space including at least one firstfluid and a second fluid immiscible with each other, the second fluidbeing electroconductive or polar; the first support plate including atleast two electrodes for applying an electric field in the pictureelement; wherein the at least two electrodes have a height difference.

When two separate electrodes are arranged in a picture element at thesame height as disclosed in international application WO 2004/104671 anddifferent voltages are applied to the electrodes, the fringing fieldsbetween the electrodes will be relatively strong and tend to break upthe layer of first fluid. It is difficult to reduce this break-up effectby reducing the distance between the two electrodes, because thereduction of distance is limited by manufacturing tolerances.Present-day techniques demand a minimum distance of 6 micrometer forhigh-end manufacturing and 15 micrometer for low-end manufacturing toavoid problems such as short-circuits. This minimum distance isrelatively large compared to the thickness of the layer of first fluidin the state without a voltage applied, which is about 4 micrometer.

The further invention solves this problem by arranging the electrodes ata different height within the first support plate. This allows thedistance between the electrodes to be set at any desired value, withoutmanufacturing problems and risks of short-circuits. The height may beformed by an intermediate layer having a thickness for example between100 and 800 nm or up to 4 micrometer. The electrodes are preferablyarranged within the display region of a picture element and excludecontrol lines; during operation the electrodes are at a substantiallyconstant voltage during a display period, i.e. a period between twoconsecutive updates of the display information of the picture element.

In a special embodiment a distance between the two electrodes in adirection parallel to the first support plate is less than a thicknessof a layer of the first fluid in a state where no voltage is applied tothe electrodes. The small distance is effective in reducing the fringingfields. When the thickness of the first fluid layer is for example 4micrometer, the distance between the electrodes should be less than 4micrometer. A direction parallel to the first support plate is alsoparallel to the plane defined above for the purpose of the heightdifferences.

When a cover layer is arranged between the two electrodes and the space,the distance is preferably less than a thickness of the cover layer. Thesmall distance reduces the fringing fields even further. When the coverlayer has a thickness of for example 800 nm, the distance should besmaller than 800 nm.

In an advantageous embodiment the electrodes overlap. The overlapreduces the fringing field further. In a preferred embodiment acapacitor may be formed in the region of overlap.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a cross-section of a part of anelectrowetting display device;

FIG. 2 shows a circuit diagram of a picture element;

FIG. 3 shows a top view of the first support plate of a picture element;and

FIGS. 4, 5, 6 and 7 show a cross-section of the first support plate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a diagrammatic cross-section of part of an electrowettingdisplay device 1. The display device includes a plurality of pictureelements 2, one of which is shown in the Figure. The lateral extent ofthe picture element is indicated in the Figure by two dashed lines 3, 4.The picture elements comprise a first support plate 5 and a secondsupport plate 6. The support plates may be separate parts of eachpicture element, but the support plates are preferably shared in commonby the plurality of picture elements. The support plates may include aglass or polymer substrate 7 and may be rigid or flexible.

The display device has a viewing side 8 on which an image or displayformed by the display device can be viewed and a rear side 9. In thefigure the first support plate 5 faces the rear side; the second supportplate 6 faces the viewing side; alternatively, the first support platemay face the viewing side. The display device may be of the reflective,transmissive or transflective type. The display device may be of asegmented display type in which the image may be built up of segments,each segment including several picture elements. The display device maybe an active matrix driven display type or a passively driven displaydevice.

A space 10 between the support plates is filled with two fluids: a firstfluid 11 and a second fluid 12. The second fluid is immiscible with thefirst fluid. The second fluid is electrically conductive or polar, andmay be water or a salt solution such as a solution of potassium chloridein a mixture of water and ethyl alcohol. The second fluid is preferablytransparent, but may be coloured, white, absorbing or reflecting. Thefirst fluid is electrically non-conductive and may for instance be analkane like hexadecane or (silicone) oil.

The first fluid absorbs at least a part of the optical spectrum. Thefluid may be transmissive for a part of the optical spectrum, forming acolour filter. For this purpose the fluid may be coloured by addition ofpigment particles or dye. Alternatively, the first fluid may be black,i.e. absorb substantially all parts of the optical spectrum, orreflecting. The hydrophobic layer may be transparent or reflective. Areflective layer may reflect the entire visible spectrum, making thelayer appear white, or part of it, making it have a colour.

A hydrophobic layer 13 is arranged on the support plate 5. The layer maybe an uninterrupted layer extending over a plurality of picture elements2 or it may be an interrupted layer, each part extending only over onepicture element 2, as shown in the Figure. The layer may be for instancean amorphous fluoropolymer layer such as AF1600 or another low surfaceenergy polymer. The hydrophobic character causes the first fluid toadhere preferentially to the first support plate 5, since the firstfluid has a higher wettability with respect to the surface of thehydrophobic layer 13 than the second fluid. Wettability relates to therelative affinity of a fluid for the surface of a solid.

Each element 2 includes an electrode 14 arranged on the substrate 7. Theelectrode 14 is separated from the fluids by an insulating cover layer,which may be the hydrophobic layer 13. The electrode 14 can be of anydesired shape or form. The electrode 14 is supplied with voltage signalsby a signal line 15, schematically indicated in the Figure. A secondsignal line 16 is connected to an electrode which is in contact with theconductive second fluid 12. This electrode may be common to allelements, when they are fluidly interconnected by and share the secondfluid, uninterrupted by walls. The picture elements 2 can be controlledby a voltage V applied between the signal lines 15 and 16. Theelectrodes 14 on the support plate 5 each are coupled to a displaydriving system. In a display device having the elements arranged in amatrix form, the electrodes can be coupled to a matrix of printedcontrol lines on the first support plate.

The first fluid 11 is confined to one picture element by walls 17 thatfollow the cross-section of the picture element. Although the walls areshown as structures protruding from the substrate 7, they may also be asurface layer on the substrate that repels the first fluid, such as ahydrophilic layer. The walls may extend from the second to the firstsupport plate but may also extend partly from the first support plate tothe second support plate. The extent of the picture element, indicatedby the dashed lines 3 and 4, is defined by the centre of the walls 17.

Further details of the picture elements of the display are disclosedamongst others in international patent application WO 03071346.

When a non-zero voltage V is applied between the signal lines 15 and 16,the picture element will enter into an on-state. Electrostatic forceswill move the second fluid 12 towards the electrode 14, therebyrepelling the first fluid 11 from at least part of the area of thehydrophobic layer 13 towards the walls 17 surrounding the area of thehydrophobic layer. When completely repelled, the first fluid may take adrop-like form as schematically indicated by a dashed line 18. Thisaction uncovers the first fluid from the surface of the hydrophobiclayer 13 of the element. When the voltage across the element is returnedto a value of zero or near to zero, the element will return to anoff-state, where the first fluid flows back to cover the hydrophobiclayer 13. In this way the first fluid forms an electrically controllableoptical switch in each picture element.

FIG. 2 shows a circuit diagram including the electronic components of apicture element of the active matrix type. The components includetransistors, capacitors and interconnecting wiring. A picture elementmay include one or more transistors. The figure shows an embodimentusing two thin film transistors (TFTs). The two TFTs 20 and 21, havegates 22 and 23, sources 24 and 25 and drains 26 and 27, respectively.The two transistors are connected in series. Gates 22 and 23 areconnected to a gate control line 30 having a voltage Vgate. The source24 is connected to a source control line 31 having a voltage Vsource.The drain 27 is connected to a capacitor Cew 32 and a capacitor Cst 33.Capacitor Cew is an electrowetting capacitor having a plate 34 formed bythe electrode 14 of the picture element in FIG. 1 and a plate 35 formedby the second fluid 12. The plate 35 is connected to a voltage Vtop,which is the voltage applied by signal line 16 to the second fluid. Thecapacitor Cst is a storage capacitor having plates 36 and 37. Plate 37is connected to a storage control line 40 to which a voltage Vstorage isapplied.

In operation a voltage to be applied to electrode 14, i.e. plate 34, isset on the source control line 31. When a selection pulse is set on thegate control line 30, the transistors 20 and 21 become conducting andthe voltage on the source line is applied to both capacitors 32 and 33.After the pulse, the resistance of the transistors increases by severalorders of magnitude and the voltage remains on the capacitors during aperiod determined by the combined capacitance and leakage currents ofthe two capacitors. Normally the voltage on the picture elements isrefreshed sufficiently often to make any effect of a decreasing voltageon the electrode 14 on the display effect of the element not noticeablefor a viewer of the display device. The purpose of capacitor 33 is toincrease the period between refreshes of the picture element. The dualtransistor allows for use of higher voltages than a single transistor.Depending on the voltages required, the switching may also be carriedout by a single transistor.

FIG. 3 shows a top-view of the first support plate 5 of a pictureelement along line A-A in FIG. 1 and as seen from the space 10. TheFigure shows one picture element 2 and part of the eight neighbouringelements. The Figure shows the walls 17 of the picture element andseveral electrically conducting elements implementing the circuitdiagram of FIG. 2. The extent of a picture element in the plane of thefirst support plate 5 is the area between the centre of the walls. Thearea between the walls, to which the first fluid is confined, is calledthe display region. The edges of an element visible from the space 10are shown by a drawn line, the edges of an element covered by anotherelement are shown by a dashed line. A typical size of the display regionis 145 micrometer by 145 micrometer. For sake of clarity the Figureprovides details according to the invention mainly for picture element 2and not for the surrounding picture elements.

FIG. 3 shows three superposed conductive layers. Although the embodimentin the Figure is a so-called field-shielded picture element, theinvention also applies to basic design picture elements. A first layer42 is closest to the substrate 7 and forms the plate 37 of the capacitor33 in FIG. 2. Its shape is rectangular with a rectangular part 43 cutout in the bottom left hand corner. A second layer 44 is also arectangular plate with a curved section, bound by a curved line 45, cutout and lying on top of the layer 42. The layer 44 forms the plate 36 ofthe capacitor 33. A third layer 46 lies above the layer 44 viewed fromthe space 10 and is a rectangular plate with a curved section, bound bya curved line 47, cut out. The layer 46 forms the electrode 14 in FIG.1, i.e. the plate 34 of the capacitor 32 in FIG. 2.

An element 50, lying under the layer 46, includes among others thetransistors 20, 21 and several connections arranged on the substrate.Layer 44 has an extension 41 connecting it to the element 50. Aconductive strip 51 forms the gate control line 30. Two branches 52 ofthe strip 51 connect the strip to the element 50. A conductive strip 53forms the source control line 31, which is connected to the element 50by a connection not shown in FIG. 3. A conductive strip 54 forms thestorage control line 40 and is connected to the layer 42, i.e. the plate37 of capacitor 33. The control lines may be arranged to the side of,above or under the layers 42, 44 and 46.

FIG. 4 shows a cross-section of the first support plate of an embodimentsimilar to that of FIG. 3 along the line B-B in FIG. 3. Thecross-section is through the element 50 comprising the transistors.

The source line 53, shown in cross-section in FIG. 4, is a conductivestrip made of e.g. a metal such as aluminium or a transparent materialsuch as indium tin oxide (ITO). The thickness of the layer may bebetween 100 nm and 300 nm. A conductive strip 55 connects the sourceline 53 to the source 24 of the transistor 20. The branches 52 and thegate line 51 may be made of a metal such as aluminium molybdenum andhave a thickness between 100 nm and 300 nm. The branches are covered byan insulating layer 56, e.g. made of silicon nitrite, SiNx, which mayhave a thickness between 150 nm and 300 nm. A layer 57 of amorphoussilicon is arranged between the layer 56 and the source 24. The drain26, preferably made of the same metal as the source 24, is arranged onthe layer 57. The transistor 21 is build up in the same way. The drain27 of the transistor 21 is connected by a metal layer 58 to a part ofthe layer 44, being one of the plates of the storage capacitor 33. Theother plate of the storage capacitor is shown as layer 42, preferablymade of a metal such as aluminium.

The transistors and the storage capacitor are covered by an insulatinglayer 59, made of e.g. SiNx or SiOx, which may have a thickness between200 and 800 nm. The insulating layer 59 is covered by a layer 60, whichmay be made of an organic material having a thickness of about 2 to 4micrometer. The electrode 46 is arranged on the layer 60 and ends at thecurved line 47. The electrode is made of a conductive material. When thedisplay device is of a transmissive type, the electrode is preferablymade of a transparent material such as ITO. When the display device isof a reflective type, the electrode may have the function of reflectorand may be made of aluminium neodymium with a thickness between 100 nmand 150 nm. The electrode is connected to the layer 58 by athrough-connection 61 through the insulating layers 59 and 60. Thethrough-connection is a local depression 62 in the electrode, such thatit is in contact with a layer 63 of conductive material, such as ITO,arranged on the strip 58. The local depression corresponds to thethickness of the layer 60.

When the electrode 46 is reflective and the walls 17 are made of atransparent material, it is undesirable that the electrode extends underthe walls, because this will give a disturbing reflection of lightincident from the side of the second support plate 6. This reflectionwill not be affected by the movement of the first fluid. The reflectioncan be avoided by not extending the electrode 46 under the walls 17, asshown in FIG. 4. When the electrode does not extend until the wall, theelectric field near the wall may be weaker than above the electrode 46or even be zero. This might cause the first fluid to stick to the wall,thereby impeding the motion of the first fluid. The field strength nearthe wall can be increased, without degrading the optical performance ofthe display, by applying a layer 64 of transparent, conductive material,such as ITO, on the electrode 46 and extending until the wall or underthe wall. The thickness of the layer may be between 30 nm and 200 nm.The layer 64 will not cause undesirable reflections from the walls,because it is transparent.

A cover layer 65 is arranged on the electrode 46 and extendssubstantially over the area of the picture element. The cover layer ispreferably a hydrophobic layer, made of e.g. AF1600, having a thicknessbetween 200 nm and 1000 nm. It may also comprise two or more layers,e.g. a protective layer of SiNx or SiOx having a thickness between 100nm and 150 nm on the electrode 46 and a hydrophobic layer of 200 nm to1000 nm on the protective layer. The cover layer has a surface 68adjoining the space 10.

The first fluid 11 is shown in FIG. 4 in the off-state. A meniscus 72between the first fluid 11 and the second fluid 12 is shown as astraight line. The meniscus can also be slightly curved upward ordownward, depending on the filling process used for the space and thetype of wall 17. In the case of an upward curvature, the thickness ofthe layer of first fluid in the off-state may vary between 4 micrometernear a wall and 6 micrometer in the middle of the picture element due tothe curvature. The total height difference of the surface 68 from leftto right in the Figure may be 400 to 800 nm.

The total effect of the height differences in the embodiment shown inFIG. 4 is that the height of the surface 68 decreases from the left handside to the right hand side, forming a recess 70, bounded by the curvedline 47 and the walls 17 in FIG. 3.

The conducting layers in the first support plate that cause an electricfield extending from that layer to the second fluid are calledelectrodes. The electrodes are separated from the space 10 only by oneor more non-conducting layers. Hence, elements 46 and 64 are electrodes;similarly, layer 44 is an electrode where it faces the space 10 withoutan intervening conducting layer. The layer 42 is therefore also anelectrode over the area without intervening conducting layer. Thestrength of the electric field over the first fluid 11 in FIG. 4 isdetermined by the distance between the meniscus 72 and the electrodes inthe first support plate and the voltage between the second fluid 12 andthe electrodes.

When a voltage is applied between the second fluid and the electrode 46,the electric field is strongest above the electrode 64 and decreases instrength to electrode 46 and even further to electrode 44, because thedistance between the meniscus and the electrodes 64, 46 and 44increases. The distance increases further for electrode 42. Since thevoltages applied to electrode 42, Vstorage, and the second fluid 12 areequal, the electric field above electrode 42 is small.

The electric field causes a stronger force on the first fluid near theleft wall than in the middle of the picture element, thereby forming apreferred direction of motion to the recess 70. In FIG. 3 this isdirection is from top left to bottom right. The recess may also bearranged in one of the other corners of the picture element, thepreferred direction of motion then being towards that corner. When therecess is an elongate area along a wall, the preferred direction ofmotion will be towards this wall. A change in distance of 5 to 10%already has an effect on the motion of the first fluid when changing thepicture element from on off-state to an on-state.

The electrodes in FIG. 4 show several height differences. The heightdifferences are at a surface 69 of the electrode that faces the space 10and determines the distance relevant for the strength of the electricfield. A height difference 73 between electrodes 64 and 46 is caused bythe arrangement of electrode 64 on electrode 46. A height difference 74between a part 75 and a part 76 of the electrode 46 corresponds to athickness of patterned layers 52 and 57, forming the transistors 20 and21. A height difference 77 between the electrodes 46 and 44 correspondsto the thickness of the layer 60 and the thickness of the electrode 46.A height difference 78 between the electrodes 44 and 42 corresponds tothe thickness of the patterned insulating layer 56 and the thickness ofthe electrode 44, the layer 56 forming the capacitor 33. The height ofthe electrodes in FIG. 3 decreases from left to right, forming a lowestregion in a recess 79, which coincides with the recess 70 in the surface68 of the cover layer. The height difference 62 of thethrough-connection 61 corresponds to thickness of the layer 60, throughwhich the connection 63 between the electrode 46 and the layer 58 ismade.

The height difference 74 of the electrode 46 corresponds to a heightdifference in the patterns of the layers 52 and 57 that caused them. Thelevelling effect of intermediate layers between the patterned layer andthe electrode, such as layer 60 may reduce the height differences. Thelevelling effect may depend on the method of deposition of theintermediate layers, such as vapour deposition or spinning.

In a prior art embodiment disclosed in international application WO2004/104671 electrodes similar to the electrodes 44 and 42 have beenarranged at the same height in the first support plate. The distancebetween the two electrodes must be at least 6 micrometer for currentmanufacturing techniques. Since the thickness of the layer of firstfluid is approximately 4 micrometer, the fringing fields between theelectrodes are relatively strong and tend to break up the layer of thefirst fluid. The fringing fields are strongly reduced in the embodimentshown in FIG. 3, where the electrodes 44 and 42 overlap, made possibleby the height difference between them. The embodiment uses this heightdifference additionally by forming a capacitor in the region of overlap.The electrodes 44 and 46 overlap in an area to the left of capacitor 33in FIG. 3. FIG. 4 shows an overlap of the electrodes 42 and 46.

The surface 68 of the cover layer will also show height differences 80and 81 that correspond to the one or more layers forming the one or moreelectronic components.

The through-connection 61 forms a depression 82 in the surface 68typically having a width of 10 micrometer and a depth of 3 or 4micrometer. The depth may be increased by changing the thickness of thelayers.

FIG. 5 shows a cross-section of the first support plate along the lineC-C in FIG. 3. The area of the storage capacitor 33 and the recess 70,79 are indicated in the figure.

The layer 42 in FIG. 5 extends beyond a first area in which the storagecapacitor 33 is formed into a second area of the recess 70. The layercan be set at the voltage Vstorage through the storage control line 54.When Vstorage is equal or substantially equal to the voltage applied tothe second fluid, the electric field over the first fluid in the recess70 will be small or zero in the on-state. Hence, when the electric fieldis applied to the picture element by applying a voltage on the electrode46, the force on the first fluid towards the recess 70 will beincreased. This increase in force due to the area with a small electricfield will be present both when the surface 68 of the first supportplate 5 shows the height differences 69 and when the surface 68 is flat.When the voltage applied to layer 42 is substantially different from thevoltage applied to the second fluid, the layer 42 may act as aninitiation point for starting movement of the first fluid away from thelayer.

The electrodes in FIG. 5 show several height differences. A heightdifference 85 is caused by arrangement of the electrode 64 on theelectrode 46. The height difference 86 is due to the patterned layer 44forming the plate 36 of capacitor 33. The shape of this heightdifference is indicated in FIG. 3 as line 45. A height difference 87between the electrodes 46 and 42 is due to the layer 60 and thethickness of the electrode 46.

FIG. 6 shows an alternative embodiment to obtain the additional electricfield in said second area. The figure shows the right-hand side of FIG.4. An additional electrode 90 has been arranged on the layer 60,covering the area of the recess 70 and possibly a further area closer tothe electrode 46. The electrode 80 is set at a voltage such that a smallor a zero electric field is generated in the recess 70. In a preferredembodiment, the voltage on electrode 90 is the same as the voltage onthe second fluid, thereby reducing the fringing fields at the curvedline 47 forming the edge of electrode 46 in the recess area. In apreferred embodiment the electrode 90 is connected to layer 42. In theFigure the connection is made by a through-connection 91, which may havea construction similar to the through-connection 61 shown in FIG. 4. Thethrough-connection 91 forms a depression 92 in the surface 68, acting asa pinning centre for the first fluid, thereby keeping the first fluid inthe recess 70.

The effect of the additional electrode 90 will be present both in anembodiment of the display device as shown in FIG. 6, where the surface68 has height differences and in an embodiment where the surface 68 isflat.

FIG. 7 shows a cross-section of a first support plate where the heightof the electrode is determined by a layer forming a control line. Thesource control line 53 is arranged adjacent to one of the walls 17 ofthe picture element. The thickness of this control line gives rise to aheight difference 94 in a surface 95 of an electrode 96. The increasedelectric field above the control line will force the first fluid awayfrom the wall when a voltage is applied to the control line, therebyreducing pinning of the first fluid on the wall. Another advantage ofincreasing the thickness of the control line is the reduction of itsresistance. These advantages can be achieved also with control linesother than the source control line.

The area having the increased field in FIG. 7 has been enlarged by alayer 97 arranged parallel to the layer 53 of the control line and madeof the same material. The layers 53 and 97 can be made in one processstep by patterning one applied layer into two separate layers. The layer97 has no function in forming an electronic component.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

The invention claimed is:
 1. An electrowetting display devicecomprising: a plurality of picture elements having a first support plateand a second support plate and a space between the first support plateand the second support plate, the space including at least one firstfluid and a second fluid immiscible with each other, the second fluidbeing electroconductive or polar; the first support plate including atleast one electrode for applying an electric field extending from thesecond fluid to the at least one electrode and a layer arranged on aside of the at least one electrode facing away from said space, thelayer forming an electronic component for applying a voltage to the atleast one electrode and the electronic component being any of a controlline, a capacitor or a further electrode, the further electrode beingarranged for applying an electric field extending from the second fluidto the further electrode; wherein the at least one electrode comprises aheight difference corresponding to a thickness of the layer anddetermining a preferred direction of motion of the first fluid in adirection of increasing distance between the at least one electrode andthe second support plate.
 2. An electrowetting display device accordingto claim 1, wherein the at least one electrode comprises a first partand a second part and the height difference is between the first partand the second part.
 3. An electrowetting display device according toclaim 1, wherein the plurality of picture elements comprises a pictureelement comprising at least two electrodes and the height difference isbetween the at least two electrodes.
 4. An electrowetting display deviceaccording to claim 1, wherein the height difference corresponds to apattern of the layer that forms the electronic component.
 5. Anelectrowetting display device according to claim 1, wherein theplurality of picture elements comprises a picture element and an extentof the picture element is defined by walls arranged in the first supportplate and the control line is arranged inside the picture elementadjacent to one of the walls.
 6. An electrowetting display deviceaccording to claim 1, wherein the plurality of picture elementscomprises a picture element and the height difference forms a recess ina corner of the picture element.
 7. An electrowetting display deviceaccording to claim 6, wherein the at least one electrode includes afirst electrode arranged in the recess and a second electrode arrangedin an area of the picture element different from the recess.
 8. Anelectrowetting display device according to claim 1, wherein the firstsupport plate includes a further layer other than said layer, the atleast one electrode comprises a height difference corresponding to athickness of the further layer.
 9. An electrowetting display deviceaccording to claim 1, wherein the first support plate includes aconducting layer arranged on the side of the at least one electrodefacing away from said space, the at least one electrode and theconducting layer being separated by an insulating layer, and at leastone through-connection between the at least one electrode and theconducting layer, the at least one through-connection forming a pinninglocation for the first fluid.
 10. An electrowetting display deviceaccording to claim 9, wherein the plurality of picture elementscomprises a picture element and the at least one electrode has a recessin an area of the picture element and the at least onethrough-connection is arranged close to or in the area.
 11. Anelectrowetting display device according to claim 1, wherein a coverlayer is arranged between the at least one electrode and the space. 12.An electrowetting display device according to claim 1, wherein theplurality of picture elements comprises a picture element, the at leastone electrode for applying an electric field extending from the secondfluid to the at least one electrode covering a first area of the pictureelement; the electrowetting display device comprising a storagecapacitor for maintaining a voltage on the at least one electrode, thestorage capacitor having two plates; wherein at least one of the platesof the storage capacitor covers a different, second area of the pictureelement, towards which the first fluid preferentially moves, the atleast one of the plates of the capacitor being at the same voltage asthe second fluid for applying a small or zero electric field in thesecond area between the second fluid and the at least one of the platesin all on states.
 13. An electrowetting display device according toclaim 12, wherein the first support plate includes a second electrodecovering at least part of the second area and connected to the at leastone of the plates of the capacitor by a through-connection.
 14. Anelectrowetting display device according to claim 1, the at least oneelectrode including at least two electrodes, each of them forindependently applying an electric field extending from the second fluidto the at least one electrode; wherein the at least two electrodes havea height difference.
 15. An electrowetting display device according toclaim 14, wherein a distance between the at least two electrodes in adirection parallel to the first support plate is less than a thicknessof a layer of the first fluid in a state where no voltage is applied tothe at least two electrodes.
 16. An electrowetting display deviceaccording to claim 14, wherein a cover layer is arranged between the atleast two electrodes and the space, and a distance between the at leasttwo electrodes in a direction parallel to the first support plate isless than a thickness of the cover layer.
 17. An electrowetting displaydevice according to claim 14, wherein the at least two electrodesoverlap.
 18. An electrowetting display device according to claim 1, anextent of the picture elements being defined by walls arranged in thefirst support plate; the at least one electrode including at least onereflective electrode not extending until any of the walls, for applyingan electric field extending from the second fluid to the electrode;wherein the at least one electrode further includes a transparentelectrode extending until or under the wall, for applying an electricfield extending from the second fluid to the transparent electrode andelectrically connected on the first support plate to the reflectiveelectrode.
 19. An electrowetting display device according to claim 7,wherein the first electrode and the second electrode have a heightdifference.
 20. An electrowetting display device according to claim 19,wherein the first electrode and the second electrode are electricallyconnected.
 21. An electrowetting display device according to claim 1,wherein a cover layer is arranged between the at least one electrode andthe space, the cover layer having a surface adjoining the space, whereinthe surface of the cover layer is non-planar.
 22. An electrowettingdisplay device according to claim 21, wherein the surface of the coverlayer is stepped.
 23. An electrowetting display device according toclaim 21, wherein the cover layer has a substantially equal thickness.24. An electrowetting display device according to claim 21, wherein thesurface of the cover layer has a height difference corresponding to thethickness of the layer forming an electronic component.
 25. Anelectrowetting display device according to claim 24, wherein adielectric constant of the cover layer and a dielectric constant of thefirst fluid are such that the height difference of the surface of thecover layer does not affect an electric field applied in the pictureelement.
 26. An electrowetting display device according to claim 25,wherein the dielectric constant of the cover layer and the dielectricconstant of the first fluid are the same.
 27. An electrowetting displaydevice according to claim 21, wherein the surface of the cover layer hasa height difference corresponding to the height difference of the atleast one electrode.
 28. An electrowetting display device according toclaim 21, wherein the plurality of picture elements comprises a pictureelement, the electrowetting display device having an off state where noelectric field is applied in the picture element, wherein when theelectrowetting display device is in the off state the first fluidadjoins the surface and has a difference in thickness corresponding tothe thickness of the layer forming an electronic component.
 29. Anelectrowetting display device according to claim 21, wherein the coverlayer is a hydrophobic layer.
 30. An electrowetting display deviceaccording to claim 21, wherein the cover layer comprises two or morelayers, one of the two or more layers being a hydrophobic layer.